Generally, a polyacrylonitrile based carbon fiber is obtained by a flame-proofing treatment at 200° C. or more in an oxidizing atmosphere and a carbonizing treatment at 300° C. or more in an inert atmosphere. In a flame-proofing step, a flame-proofing reaction is started when the temperature of a precursor fiber bundle is increased by hot air. Further, the flame-proofing reaction is controlled by removing the reaction heat of the flame-proofing reaction. Here, when the wind speed or the temperature in the hot air is not uniform, the flame-proofing reaction would not be uniform, and hence troubles such as smoke or breakage of fibers occurs. Further, the quality of the product is not uniform. Therefore, there is a need to remove unevenness in flame-proofing step by performing a flame-proofing treatment in a uniform condition, and thus to attain a uniform property of an obtained continuous fiber bundle and improving production efficiency.
In the related art, a heat treatment furnace, especially, a heat treatment furnace used to produce a carbon fiber includes a blowout nozzle which blows hot air into a heat treatment chamber supplied from a circulation fan. Here, it is desirable that the velocity of the hot air supplied from the blowout nozzle into the heat treatment chamber be uniform. For example, JP 58-208433 A (Patent Document 1) discloses a blowout nozzle in which a hot air blowing surface is provided so that hot air blows along a traveling yarn, a direction changing guide vane is provided in the hot air blowing surface so as to direct the hot air, and a metal mesh or a porous plate as a flow conditioner (rectifier) is disposed at one of the front and rear sides of the direction changing guide vane or both front and rear sides thereof. According to this method, when the average wind speed of the heat treatment chamber is 2 m/s, the variation can be adjusted within 1.5 to 2.5 m/s.
Further, JP 2002-194627 A (Patent Document 2) discloses a blowout nozzle having a uniform wind speed distribution in the width direction of a nozzle blowout port as below. The inside of the blowout nozzle is defined as an introduction zone and a flow rectification zone, and the introduction zone is provided with a guide vane that decreases bending loss in a passage. In the flow rectification zone, porous plates are inserted into the nozzle in the direction substantially perpendicular to the hot air flowing direction and a space is formed at the downstream side of the each porous plate. Accordingly, it is possible to exhibit an effect of decreasing the unevenness of wind speed of the hot air. Further, the flow of the hot air is rectified in a direction perpendicular to the nozzle blowout port by a plurality of rectification plates provided right before the blowout port of the nozzle. In order to make the difference ΔV between the maximum wind speed and a minimum wind speed in the width direction of the nozzle blowout port be within Vm, the number of stages of the flow rectification zone is set so as to fix the pressure loss of the flow rectification zone by setting the number N of the stages of the flow rectification zone provided inside the nozzle to be ½ or more of λ/Vm based on a coefficient λ. Accordingly, the wind speed is controlled within the range of 2.9 to 3.2 m/s with respect to the average wind speed of 3 m/s of the treatment chamber.